Add warning about publicly exposing usernames.

[SijmenHuizenga]

Hi! Awesome work with this plugin, we use it all the time and it works superbe!

I have just one suggestion: add a notice to the top of the readme, wiki and atlassian-plugin-page that warns administrators that by default, usernames will be exposed to the public. Maybe something along the lines of:

Be aware, this plugin publicly exposes some sensitive data by default. The metrics page exposes the username of who last logged in, who last edited issues in every project and some more usernames. You should enable token protection to make sure the public cannot view usernames.

[AndreyVMarkelov]

Will do